In the course of manufacturing paper and similar products, including paperboard and the like, it is well-known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. In the absence of such "fillers", the resulting paper can have a relatively poor texture due to discontinuities in the fibrous web. The said fillers are also important in improving the printing qualities of the paper, i.e. by improving the surface characteristics of same. The use of appropriate such fillers, further, vastly improves the opacity and the brightness of a paper sheet of a given weight.
A number of inorganic materials have long been known to be effective for many of the aforementioned purposes. Among the best of these materials is titanium dioxide, which can be incorporated into the paper in the form of anatase or of rutile. Titanium dioxide, however, is among the most expensive materials which are so useable. Thus despite the effectiveness of such material as a filler, its use is limited and satisfactory replacements have been much sought after.
Among the materials which have found increasing acceptance as paper fillers are calcined kaolins. Materials of this type are generally prepared by calcining a crude kaolin which may have been initially subjected to prior beneficiation steps in order to remove certain impurities, e.g. for the purpose of improving brightness in the ultimate product. Reference may usefully be had to Proctor U.S. Pat. No. 3,014,836, and to Fanselow et al, U.S. Pat. No. 3,586,823, which disclosures are representative of the prior art pertinent to calcined kaolins.
Those properties which render a kaolin pigment particularly valuable for use as a filler are also well-known. These include a low abrasion value, and high brightness and opacifying characterisitics. The low abrasion is significant in order to assure that the resultant paper product may be manufactured and processed using conventional machinery without damaging same. The brightness and opacifying characteristics are important in producing an acceptable paper sheet, one which incorporates whiteness, high opacity, good printability, and light weight.
Both the brightness characteristics of the given kaolin and the opacifying properties of same when incorporated as a filler in paper, may be quantitatively related to a property of the filler identified as the "scattering coefficient S". The said parameter, i.e. the scattering coefficient S of a given filler pigment, is a property well-known and extensively utilized in the paper technology art, and has been the subject of numerous technical papers and the like. The early exposition of such measurements was made by Kubelka and Munk, and is reported in Z. Tech Physik 12:539 (1931). Further citations to the applicable measurement techniques and detailed definitions of the said scattering coefficient are set forth at numerous places in the patent and technical literature. Reference may usefully be had in this connection, e.g. to U.S. Pat. Nos. 4,026,726 and 4,028,173. In addition to the citations set forth in these patents, reference may further be had to Pulp and Paper Science Technology, Vol. 2 "Paper", Chapter 3, by H. C. Schwalbe (McGraw-Hill Book Company, N.Y.).
It is well-known that in order to obtain high light scattering and opacity, the major portion of filler should be in the range of one micrometer. However, good light scatter cannot be achieved solely by using a kaolin having the said size characteristics: an essential further characteristic needed is that the kaolin be structured, i.e., formed from an assemblage of platelets interconnected or bonded to provide aggregates which include a network of the platelets.
Aggregation can be of interest for additional reasons. In particular, many kaolin crude reserves are considerably finer than preferred by the paper industry; i.e., they have an unduly high proportion of particles with E.S.D.'s below 0.25 micrometers. Typical Cretaceous kaolins, e.g., include 25 to 30% by weight of particles below 0.25 micrometers; and typical Tertiary kaolins can include 50 to 60% by weight of particles below 0.25 micrometers E.S.D. The presence of such large quantities of very fine particles can have detrimental effects upon paper strength, and in other respects are undesired by the paper manufacturer. While the said extreme fines can be removed by various separation processes, such as high speed centrifuging, such removal is very expensive and entails high capital investment; this quite aside from the possible loss of high proportions of the input feed.
One method for achieving aggregation is to utilize calcining. Thus, in U.S. Pat. No. 4,381,948 to A. D. McConnell et al, a calcined kaolin pigment is disclosed and a method for manufacture of same. The said pigment consists of porous aggregates of kaolin platelets, and exhibits exceptionally high light scattering characteristics when incorporated as a filler in paper. This pigment, which substantially corresponds to the commercially available product ALPHATEX.RTM. of the present assignee Anglo-American Clays Corporation (Atlanta, Ga.), is prepared by first blunging and dispersing an appropriate crude kaolin to form an aqueous dispersion of same. The blunged and dispersed aqueous slurry is subjected to a particle size separation from which there is recovered a slurry of the clay, which includes a very fine particle size; e.g. substantially all particles can be smaller than 1 micrometer E.S.D. The slurry is dried to produce a relatively moisture-free clay, which is then thoroughly pulverized to break up agglomerates. This material is then used as a feed to a calciner; such feed is calcined under carefully controlled conditions to typical temperatures of at least 900.degree. C. The resulting product is cooled and pulverized to provide a pigment of the porous high light scattering aggregates of kaolin platelets as described.
Calcined kaolins have also found use in paper coating applications. Reference may be had to the paper by Hollingsworth, Jones, and Bonney, "The Effect of Calcined Clays on the Printability of Coated Rotogravure and Offset Printing Papers", TAPPI Proceedings, pages 9-16, 1983 Coating Conference, discussing the advantages of incorporating small quantities of calcined kaolins into conventional kaolin-based coating formulations. Brightness and opacity of the paper both increase with increased calcined kaolin content, as may be expected from a pigment with high light scatter, and in some formulations gloss may show a slight increase with increasing calcined kaolin content.
Calcined kaolin products, including those of the aforementioned ALPHATEX.RTM. type, are seen to be manufactured by relatively complex techniques involving a multiplicity of steps, including specifically a calcining step, plus various preparatory steps and post-calcining steps. Thus, the said product is relatively expensive to produce; and requires considerable investment in complex apparatus and the like--e.g. highly regulated calciners, etc. It can indeed be noted that the conditions of preparation of these materials must be very carefully controlled in order to keep abrasion acceptably low in the calcined product. For example the calcination operation tends per se to produce an abrasive product--in consequence of overheating--if great care is not taken to preclude such a result.
It is further to be noted that in order to produce a low abrasion calcined product, the particle size in the feed to the calciner must be carefully controlled--even a relatively small increase in coarseness of such feed can have very marked detrimental effect on Valley abrasion.
It has heretofore been known to utilize uncalcined (sometimes referred to as "hydrous") kaolin both as paper fillers and for paper coating. Because the uncalcined material usually does not possess high light scattering qualities or good opacity, its usefulness, especially as a filler, is limited; and this (in addition to improving brightness) is indeed the particular advantage of calcined products of the ALPHATEX.RTM. type; i.e. by virtue of the aggregated structures of same, high light scattering properties are provided and good opacity.
With respect further to terminology, it is noted that the prior art literature, including numerous of the prior art patents relating to the field of kaolin products and processing, often uses the term "hydrous" to refer to a kaolin which has not been subjected to calcination--more specifically, which has not been subjected to temperatures above about 450.degree. C., which temperatures serve to impair the basic crystal structure of kaolin. These so-called "hydrous" clays may have been produced from crude kaolins, which have been subjected to beneficiation, as, for example, to froth flotation, to magnetic separation, to mechanical delamination, grinding, or similar comminution, but not to the mentioned heating as would impair the crystal structure.
In an accurate technical sense, the description of these materials as "hydrous" is, however, incorrecr. More specifically, there is no molecular water actually present in the kaolinite structure. Thus, although the composition can be (and often is) arbitrarily written in the form 2 H.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2, it is now well-known that kaolinite is an aluminum hydroxide silicate of approximate composition Al.sub.2 (OH).sub.4 Si.sub.2 O.sub.5 (which equates to the hydrated formula just cited). Once the kaolin is subjected to calcination, which, for the purposes of this specification means being subjected to heating of 450.degree. C. or higher for a period which eliminates the hydroxyl groups, the crystalline structure of the kaolinite is destroyed. Therefore, such material, having been thus calcined, cannot correctly be referred to as a "kaolin". Accordingly, it should be appreciated that henceforth in this specification, when the term "kaolin" or "kaolinite" is utilized, such term necessarily implies that the original structure of the material is intact. Thus, the term "kaolin" as used herein, can be considered to be equivalent to the technically inaccurate (but oft-occurring) prior art usage, "hydrous kaolin" or sometimes simply "hydrous clay."
From time to time, it has been proposed to provide structured kaolin agglomerates by methods unrelated to calcining, the objective being to produce a high light scattering pigment, one with good opacifying properties, without the need for calcination.
Thus, in U.S. Pat. No. 4,346,178 to Peter Economou, a structured kaolin agglomerate is disclosed wherein the clay platelets are stabilized or frozen in position by the addition thereto of a urea-formaldehyde prepolymer.
Further relevant art includes U.S. Pat. No. 4,072,537 to F. L. Kurrle. Disclosed therein is a composite silicate pigment prepared by a precipitation reaction where spherical hydrous metal silicate particles are precipitated on the planar surfaces of clay particles having a platelet-type structure. The metal silicate pigment component is comprised of the reaction product of a water soluble alkali metal silicate such as sodium silicate and a water soluble salt of a polyvalent metal, such as calcium chloride.
In U.S. Pat. Nos. 3,853,574; 3,864,140; and 3,856,545, all to Thomas H. Ferrigno, pigmentary compositions are disclosed wherein comminuted minerals such as kaolins are combined with inorganic binders, and formed into agglomerates, which when calcined produce compositions embodying numerous particles of the minerals bonded together and presenting internal voids and multiple light reflecting surfaces. The resulting pigmentary compositions are useful in applications requiring high visible light reflectance. These products, however, are, as indicated, produced by calcining, and based upon inorganic binders such as a glassy material, e.g. an alkali metal silicate or the like.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a structured kaolin pigment product, which possesses improved light scattering characteristics, and hence is useful as an opacifier and light scattering filler for paper and paper products, and which may similarly be used in other paper manufacturing applications, including in the coating of same.
It is also an object of the invention to provide improved coated papers and paperboard products for printing purposes such as for offset printing, gravure printing and other types of printing, using a structured kaolin pigment.
It is a further object of the present invention, to provide a pigment product of the foregoing character, which is prepared without calcination and therefore without subjecting the kaolin to high temperatures, and which accordingly possesses low abrasiveness in accordance with the kaolin feed from which it is produced.
It is a yet further object of the invention, to provide a pigmentary product of the foregoing character, which is composed of structured aggregates of minute kaolin particles which are bonded together chemically, i.e. without the use of calcination or other high temperature techniques.
It is a still further object of the invention to provide a high light scattering pigmentary kaolin product, which may be produced at lower costs than calcined kaolins, and which can therefore provide a lower cost replacement for many applications in which calcined kaolin products or other high cost fillers are presently utilized.
It is a yet further object of the invention, to provide a process for producing a pigmentary high light scattering kaolin product, which process utilizes chemical aggregation techniques, and may be carried out without the use of a calcining step or of other high temperature processing conditions or equipment.
It is a yet further object of the present invention, to provide a process for producing structured kaolin pigments as aforementioned, which consists of a minimal number of simply conducted steps, which utilize relatively simple and inexpensive apparatus, and which introduce minimal foreign elements into the final product produced by same.
A still further object of the invention is to provide a process for producing structured kaolin pigments as aforementioned, from a feed crude containing high percentages of extremely minute particles, e.g., less than 0.25 micrometers, to thereby convert a crude having limited value to the paper maker to one which can be readily and economically used in paper products.
A yet further object of the invention, is to provide a process as aforementioned, which enables low abrasion structured kaolin pigments to be produced from coarser process feeds than can normally be employed in calcination to produce a product of comparable low abrasion.
It is a further object of the invention to provide bulky structured kaolin pigments which are eminently suitable for paper coating applications.
It is a still further object of the invention, to provide kaolin pigment-based coating compositions and related methods, which in comparison to use of conventional kaolin pigment-based coating compositions, improves the coating structure and coverage in papers coated with such compositions, as shown by optical properties such as gloss, brightness and opacity, and as shown by printability properties such as offset print gloss, ink receptivity, better K & N and less mottle, and fewer missing gravure ink dots.
A yet further object of the invention, is to provide improved papers of the type provided with a coating composition comprising a kaolin pigment and an adhesive binder, which papers, when manufactured pursuant to the invention, have improved optical and printability properties over a very wide range of coating weights.
It is a further object of the invention to provide paper and paperboard coating compositions which under calendaring conditions used with conventional coating compositions provide improved gloss, and alternatively with reduced calendaring, result in less coating structure collapse than with conventional coating compositions, and which compositions, further, provide smoother surfaces on papers coated therewith.
A still further object of the invention, is to provide improved paper and paperboard coating compositions, which so enhance coverage in relation to coating weight, as measured by gloss and other optical properties in resultant coated papers, as to enable a significant reduction in the coating weight, while yet maintaining the coverage, i.e. the gloss and other optical properties.